The quality of the scans used for OCR is one of the two factors deciding the quality of the output of OCR4all (the other being font training). Since I have mostly ocr’ed incunables and early 16th-century prints, this has been a major concern. ‘My’ books are often heavily discolored and hardly ever equally illuminated, since pages towards the spine often curve away from the camera and thus reflect light differently from the rest of the page. This usually does not impact the legibility of the original scans, but bitonal conversions often turn the beginning of the line into a black blob that even the ingenuity of OCR4all cannot make sense of. I have tried to improve the legibility of the scan by heightening the contrast, changing the histogram curve with Adobe Lightroom, setting the scanning parameter in OCR4all to greyscale instead of bitonal (not a success in my few attempts), creating bitonal output with Scantailor (only a success if the scan is rather uniform), etc. This blog is about the circuitous route to create optimized input for OCR4all.